Sound-reproducing apparatus



April 2, 1929. E.VV.KE..L GG ,707,611-

SOUND REPRODUCING APPARATUS Original Filed Jams}, 1925 Inventor; EdwardW Kellogg,

His Attorneg.

Patented Apr. 2, 1929.

UNITEDVSTA'IZES P TENT OFFICE.

EDWARD W. KELLOGG, OF SCI-IENECTADY, NEW YORK, ASSIGNOR TO GENERAL ELEC-TRIO COMPANY, A CORPORATION OF NEW YORK.

SOUND-BEPRODUCING'APPARAT'OS.

Application filed January 9; 1925, Serial No. 1,433 Renewed May 18,1927.

My present invention relates to apparatus for producing sound fromelectric currents corresponding to original sounds, and moreparticularly to devices now commonly known as loudspeakers. 1

' .One of the objects of my invention is to provide a device of theclass mentioned whereby the reproduction of sounds may be made withgreater accuracy and fidelity than with devices of this class previouslyemployed.

' A' further object of my invention is to provide an apparatus in whichthe usual horn may be dispensed with and the sound projected from thedevice in a more satisfactory manner than with the usual form of horn.

The novel features which I believe to be characteristic of my inventionare set forth with particularity in the appended claims.

My invention itself, however, both as to its organization and method ofoperation will best be understood by reference to the followingdescription taken in connection with the accompanying drawing-in whichFig. 1 is a view-partly in cross-section of a device constructed inaccordance with my invention; Fig. 2 is an enlarged cross-section detailshowing the method of supporting the outer edge of the diaphragm; Fig. 3is an enlarged cross-sectional detail showing the manner I in whichthe-actuating means is supplied to the diaphragm, and Fig. 4 is aperspective exploded view of the arrangement shown in Fig. 3.

I have indicated in the drawing a sound reproducing device comprising adiaphragm 1 of conical form which, in the device illus trated, isspecifically a truncated cone. The outer edge or base of the diaphragmis securedto a supporting ring 2 by means of a ring 3 of flexiblematerial such as silk, rubber, or thin leather. The diaphragm 1 may bemade of' paper or other light material whereby the diaphragm will bemade rigid enough so'that it will be capable of vibrating substantiallyas a whole. The'top or inner edge of the diaphragm is made in the formof a cylinder 4 which is provided with inner and outer reinforcing rings5 and 6. The

actuating means for the diaphragm comprises a coil 7, surrounding theinner reinforcing ring5, and secured thereto in any suitable manner. Thecoil 7 is disposed in the annular air gap 8 which is formed between theconcentric pole pieces 9 and 1( The inner pole piece 9 is surrounded bya 0011 11 through which a steady direct current ma be supplied forproducing astrong I magnetic field in the annular air gap 8.

When variable currents are supplied to the coil 7 the diaphragm 1 willbe caused .to vibrate substantially as a whole in accordance tyi lithtllie variationsin current supplied to the ring 3 there may be atendency for lat- Because of the flexible nature of vided at its centerwith a pro ection 10 and a spider 11 is secured to this extension bymeans of the screw 12. This spider is provided w1th a plurality ofresilient arms 13 extending in directions substantially parallel to theaxis of the pole piece 9. These arms 13 are all connected by radiallyextending connecting links 14 with the top of the diaphragm.. Theseconnections are located in a plane substantially parallelwith the baseof the diaphragm and may be made part of a continuous length of cord orfine wire threaded through holes in the ends of the arms 13 and throughthe end of the diaphragm, as indicated. 'Whenthese connections are madetaut they will substantially prevent all, lateral movement of the coil7, and at the same time will not interfere to any appreciable extentwith longitudinal movement of the coil in air gap 8. n

One feature which is essential to the satisfactory operation of a loudspeaker is the avoidance of resonant air chambers next to the diaphragm.For example, if the space behind the diaphragm is completely enclosedthere will be a certain frequency at which the enclosed air will furnishan elastic restoring force to the diaphragm causing a strong resonanceeffect and other frequencies at which the air will so resist motion ofthe diaphragm as to greatly reduce its amplitude of vibration. Toeliminate any enclosed air space between the diaphragm and the polepieces of the polarizing magnet the top of the cone is left open. Theclearance between the top ofthe cone and the pole pieces may be madesmall enough so that in spite of the open end construction there will beno great loss due to leakage around the inner edge .of the diaphragm.

The supporting ring 2 may be mounted on the pole piece by means ofbrackets 15.

support 17.

The magnetic structure may be secured by suitable brackets 16 to anydesired form of The restoring force of the diaphragm mountedin the wayillustrated may be made so low that its natural rate of vibration as awhole will be below the lowest important voice frequency. For example, Ihave constructed paper diaphragms of the form indicated of a diameter offive and inches and have found that when mounted in the mannerdescribed'these diaphragms have a natural rate of vibration as a wholeof about 50 cycles. Fairly ood results-may be obtained however withiaphragms having higher natural frequencies of vibration, even up to 200cycles, particularly if provision is made for damping so that theresonance is not sharp. This low natural frequency is an essentialfeature in the construction of loud speakers in order that the diaphragmshall respond equally to all frequencies which it is desiredtoreproduce.

Both sides of the diaphragm -1 are exposed to the air. As a result noresonant effects will be introduced by reason of confined air. VV'iththe arrangement thus far described the air could circulate between thefront and back of the diaphragm around the supporting ring 2 withoutundergoing any appreci able compression. This would result 1nsubstantial loss of the low tones. If, however, the path between thefront and back of the diaphragm is lengthened to a distance of about aquarter wave length or more, this circulation will be prevented and goodradiation may be secured. This effect may be had by surrounding thediaphragm by a bafile 18 of fairly massive or rigid material in such amanner as to effectively prevent circulation. The baffle effect mayconveniently be obtained by enclosing the. diaphragm and its actuatingmechanism in a cabinet of suitable form, which cabinet is provided withan opening in which the supporting ring 2 is fitted. Preferably the backof this cabinet should be open to prevent any resonant effects due toconfined air.

' The type of loud speaker which I have described and illustratedrequires for its op eration a. comparatively strong magnetic field. andthis necessitates supplying to the exciting coils 11 a current ofconsiderable strength. In some cases it may be desirable to produce thefield by means of rectified alternating current. Unless an expensivefilter system is provided the rectified current will normally be apulsating current but the magnetic field in the air gap must bepractically constant in order to avoid modulation of the tones producedby the loud speaker and the production of tones which are harmonics ofthe frequency supplied by the rectifier. This difficulty may be avoidedby making the end 19 of the field coil spool constitute a shortcircuited turn of very low resistance, for example, by making it of aheavy ring of copper.

While it may be shown theoretically that a small diaphragm, of suchrigid construction that all arts have equal motion even at the highestrequencies, and so supported that its natural frequency of Vibration asa whole is below the lowest frequency to be radiated, constitutes anideal sound reproducer, as explained in my copending application Serial-No. 702,455, filed March 27, 1924,, I have not found it necessary inpractice to adhere to diaphragms of such small size as would be neededto comply with the condition just mentioned. For example, I have made acalculation which indicates that a paper cone, four inches in diameterat the base, free at the outer edges, and with an angle of 45 betweenaxis and element will probably begin to depart materially from trueplunger action at a frequency between 3000 and 1000 vibrations persecond. Cones with larger diameters or with larger angles betweenelement and axis will begin to show appreciable phase and amplitudedifferences between the different parts of their surfaces at lowerfrequencies. I have nevertheless obtained good results withv cones up to8 and even 20 inches in diameter and with angles as great as 64,provided that in all cases the outeredges of the cone is so flexiblysupported that at low fretype of motion in which all parts of thesurface have the same motion may be described as piston or plungeraction. In the devices built in accordance with my invention, trueplunger action takes place at the lower frequencies and wave actionpresumably takes place at the higher frequencies. The transac-: tion isnot abrupt, nor is it marked by any great change in radiationcharacteristics or efficiency which would upset the balance be tween thehigh and low frequency components orimpair the general quality of soundreproduction. \Vithin the freq-uencv range characterized by wave actionin the dia- I phragm slight resonance may occur, for the reason thatreflection of the waves takes place at the outer edge of the diaphragm.The fact that resonances do not appear to be so sharp or strong as toimpair sound reproduction is probably due to the high attenuation of thewaves at high frequency by energy ab sorption in the material of thediaphragm, and also in part to energy absorption in the flexible ring bywhich the outer edge of the diaphragm is supported.

As compared with diaphragms so small that one can be sure that they actasplungers throughout the entire essential frequency range, I find again in sensitivity from the use of somewhat larger sizes, my bestresults having been obtained with diameters between four and eightinches andangles ment and axis, from to Diaphragms of large diameterwork best with the larger angles. In the case of sound reproducingdevices employingconical diaphragms, either rigidly supported at theouter edge, or with a support Whose stiffness in the axial direction iscomparable with the stiffness of the diej phragm itself, plunger actioncannot take lace at any frequency. If a vibratory force is applied at ornear the vertex the resulting motion is greatest where the force isapplied and becomes less as the edge is approached if the frequency isbelow the fundamental resonances. As the frequency is raised a series ofresonances occur. At certain frequenciesthe motion is greatest at thepoint of application of the driving force, and at other frequencies themotion is greatest at oints between the vertex and the outer edge.

lexural waves originating at the driving point, travel to the outer edgewhere theyare reflected and produce standing waves with circular nodesand loops of motion. At low frequencies the resonances are far apart onthe musical scale, and owing to the small absorption of energy,resonanceis sharp. Both of these facts mean serious distortion in thereproduction of sounds. At higher frequencies the resonances are closertogether on the musical scale and are less sharp because of greaterrelative energy absorption in the material of the diaphragm and soundradiation is therefore more nearly uniform. vWith a diaphragm stifflysupported at the edge, there is little motion and little sound radiationat frequencies below the first resonance. Therefore to secure anywherenear adaquatev radiation of low tones it is necessary to make thediameter large and the angle between 'ele-' ment and axis large (givinga shallow cone)."

My tests indicate however that cones which are very shallow incomparison with their diameters, fail to give adequate radiation of thevery high frequency tones.

A device, employing a flexibly supported conical diaphragm, has thefollowing advantages over previous devices having rigid or stiff supportaround the outer edge. The radiation of the low tones is much morenearly uniform and free from resonance, and the radiation of highfrequencies is more satisfactory for the reason that we have morelatitude inthe choice of cone angle and can choose that which gives themost nearly uniform radiation of the high frequencies. Devices withst-ifily supported outer edges must between eleemploy an angle which isa compromise between what is best forthe high and low frequencies and istherefore not the best angle for either one. My device has the furtheradvantage that with plunger diaphragm action adequate radiation of lowtones can beobtained with a smaller diaphragm, provided that a bafile isused in connection with the diaphragm. This means that the diaphragm iseasier to )rotect from injury and the resulting device ecomes morecompact.-

In the claims which follow I'use such expressions as adapted to vibratesubstantially as a Whole and all parts thereof will have substantiallyequal movement? to distinguish over diaphragms which are rigidlysupported and therefore incapable of vibration as a whole at anyfrequency, and to include diaphragins such as described which may have aslight wave action at the higher frequencies but in which such action isnot detrimental.

Certain features disclosed herein but not claimed are disclosed andclaimed in my copen'ding application Serial No. 7 02,455, filed March27, 1924. Certain other features also disclosed herein but not claimedare disclosed and claimed in the joint application of Che's- 2. In anapparatus forsound reproduction,

a diaphragm having the form of a truncated cone which is open at itstop, and which is so supported that it is adapted to vibratesubstantlally as a whole, reinforcing means for the top thereof, andmeans for actuating said diaphragm comprising a coil secured to saidreinforcing means.

3. In an apparatus for sound reproduction, a diaphragm having the formof a truncated cone which is open at the top and which is so rigid incomparison with its size and weight that when a driving force is appliedthereto at its top all parts thereof will have substantially equalmovement, means for suspending said diaphragm havingso low a restoringforce that its natural rate of vibration as a whole will be below thelowest important voice frequency, and means for actuating said diaphragmcomprising a coil surrounding the top thereof.

4. In an apparatus for sound reproduction, a diaphragm having the formof a truncated cone which is open at its top and which is so rigid incomparison with its sizeand weight that when a driving force is appliedthereto at its top all parts thereof will have substantially equalmovement, means for suspending Y top of said diaphragm, and means foractuating said diaphragm comprislng a cell secured =1 to said relnforcmgmeans.

5. In an apparatus for sound reproduction,

a diaphragm having the formof a truncated cone which is open at its topand which'is so rigid in comparison With its size and weight that when adriving force is applied thereto at its top'all parts thereof Will havesubstan tially equal movement, means for suspending said diaphragmhavingso low arestoring force that its natural rate of vibratlon as a Wholewill be below the lowest important voice frequency, means for actuatingsaid diaphragm comprising a pair of concentric pole pieces separated byan annular air gap, and an actuating coil secured to the top of saiddiaph ragm and located in said air gap. v

6. In an apparatus for sound" reproduction, a rigid diaphragm having theform of a truncated cone which is open at the top, the base of saiddiaphragm being so flexibly supported that the diaphragm is adaptedtovibrate substantially as a whole, means for actuating said diaphragmcomprising a pair of concentric pole pieces separated by an annular airgap and an actuating coil secured to the top of said diaphragm andlocated in said air a r h In an apparatus for sound reproduction, adiaphragm having the form of a truncated cone which is open at its topand which is so supported that it is adapted to vibrate substantially asa Whole, and means for actuating said diaphragm comprising a coilsurrounding the top thereof, said diaphragm being so arranged that'aircan circulate freely on both sides thereof, and a battle surroundingsaid dia hragm for increasing the length of the pat 1 through the airbetween the two sides of the diaphragm.

8. In an apparatus for sound reproduction, a rigid diaphragm having theform of a truncated cone which is open at its top, the base of saiddiaphragm being so flexibly supported that the diaphragm is adapted tovibrate as a'whole, means for actuating said diaphragm comprising a pairof concentric pole pieces separated by an annular air gap, an actuatingcoil secured to the top of said diaphragm and located in said air gap,and means secured to the inner of said pole pieces for preventinglateral movement of said coil in said air gap.

9. In an apparatus for sound reproduction, a rigid diaphragm havingtheform of a truncated cone, the base of said diaphragm being soflexibly supported that the diaphragm is adapted to vibrate as a- Whole,means for actuating said diaphragm comprising a pair of concentric polepieces separated by an annular air gap, an actuating coil secured to thetop-of said diaphragm and located in said air gap, and means forpreventia truncated cone, the base of said diaphragm I being so flexiblysupported that the diaphragm is adapted to vibrate as a Whole, means foractuating said diaphragm compris ing apair of concentric pole piecesseparated} by an annular air gap, an actuating coil secured to the topof said diaphragm and located in said air gap, and means for preventinglateral movement of said coil in said air gap, comprising a'spidersecured to the inner of said pole pieces, said spider having a pluralityof flexible arms, and each of said arms being connected by radiallyextending connections to the top of said diaphragm, said connectionsbeing taut in the direction of latteral movement of said coil butflexible in the direction of longitudinal movement thereof. v

11. In an apparatus for sound reproduction, a rigid diaphragm having theform of a truncated cone, the base of said diaphragm being so flexiblysupported that the .diaphragm is adapted to vibrate as a Whole, meansfor actuating said diaphragm comprising a pair of concentric pole piecesseparated by an annular air gap, an actuating coil secured to the top ofsaid diaphragm and located in said air gap, and means for preventinglateral movement of sa d coil in said air gap comprising aspidcr'secured to the inner of said pole pieces, said spider hav ing aplurality of flexible arms extending in directions substantiallyparallel to the longitudinal axis of said diaphragm, and each of saidarms being connected by radially extending connections to the top ofsaid diaphragm.

12. In an apparatus for sound reproduction. a rigid diaphragm having theform of a truncated cone which is open at its top, the base of saiddiaphragm being so flexibly supported that the diaphragm is adapted tovibrate as a whole, means for actnatingvsaid diaphragm comprising a pairof concentric pole pieces separated by an annular air gap, anactuating;- coil secured to the top of said diaphragm and located insaid air gap, and

means secured to the inner of said pole pieces for preventing lateralmovement of said coil in said air gap. 1 a

13. In an apparatus for sound re reduction, a rigid diaphragm having theorm of a truncated cone open at the top and a plurality of supportingIneans for said diaphragm located in substantially parallel planes nearthe base and top thereof, said supporting means being so flexible as topermit vibration of the diaphragm substantially as a Whole.

14. In an apparatus for sound reproduc-' tion, a rigid diaphragm havingthe form of a truncated cone which is open at its top, the base of saiddiaphragm being so flexibly supported thatthe diaphragm is adapted tovibrate as a whole, means for actuating said diaphragm comprising a'pair of concentric pole pieces separated by anannular air gap, anactuating coil secured to the top of said diaphragm'and located in saidair gap, and" flexible supporting means for said diaphragm located in a,plane substantially parallel with the-base of said diaphragm and.

secured-to the inner of said pole pieces. m

15".. In; an apparatus -forsound reproduchaving so low a restoring forcethat its natural rate of vibratlon as a whole will be below thelowcst'important voice frequency,

means for actuating said diaphragm comprising a pair of concentric polepieces separated by an annular air gap, an actuating .coil secured tothe top of said diaphragm and located insaid' air gap, and means securedto the lnner of said pole pieces for preventing lateral-movement of saidcoil in said air gap.

In witness whereof, I have hereunto set my hand this 8th day of January,1925. v

EDWARD W. KELLOGG.

